Civilians were mobilized Friday to join exhausted soldiers and emergency workers struggling against mounting difficulties to retrieve thousands of chemical-filled barrels that were swept into a major northeast China river by flood waters two days ago.

Salvage workers fear some of the barrels, many filled with 170 kilograms of flammable liquid, may have sunk to the bottom of the Songhua River, raising serious risks of lingering water contamination.

Chemical barrels were also spotted lying unattended in the debris of flood-devastated villages.

Some 3,000 full barrels and 4,000 empty ones were swept into the Wende River and on to the Songhua River after floods hit warehouses of two chemical factories in Jilin City, Jilin Province, early Wednesday.

By noon Friday, 2,978 barrels had been recovered, but workers say floating debris, such as trash, weeds and tree branches, have hampered the work.

On Friday, the local government encouraged the public to join in the operation, announcing a bounty of 100 yuan for each full barrel and 50 yuan for each empty one that local people brought in.

It was not clear whether professional handling was needed for the chemicals, but a local official said many riverside residents "had experience in salvage."

Soldiers and emergency workers stationed at eight points on the waterways in Jilin Province have worked around the clock since late Wednesday to collect the barrels before the torrents carry them further down the Songhua River into neighboring Heilongjiang Province.

At one point, where boats were chained together across the river, engineers used four cranes to remove debris before soldiers and experts on boats used long poles and steel nets to retrieve the barrels.

Heilongjiang on Friday also sent soldiers and workers to six points on the lower reaches of the river as water resources experts predicted the flow might carry the first barrels into the province late Friday.

However, helicopter spotters who have been tracking the barrels have lost sight of some of them. Experts fear they may have sunk into the river -- making their retrieval more difficult.

A video posted on Chinese web portal Sina.com Thursday showed a barrel catching fire and exploding on the river's surface.

Of the 3,000 chemical-filled barrels, about 2,500 contain trimethyl chloro silicane -- a colorless flammable liquid -- while 500 contain hexamethyl disilazane, also a colorless liquid, officials earlier said.

"The chemical will not cause havoc on the river unless a large number of containers are damaged at one time," said Sun Lili, deputy general engineer with the Design and Research Institute of Petrochemical Technology in Jilin Province.

National and local environment watchdogs have set up stations along the river to test the water quality.

A test conducted early Thursday showed the water was not contaminated. Results of water quality tests conducted Friday are not yet available.

The incident has revived memories of the contamination of the Songhua River in 2005 after an explosion at a petrochemical plant in Jilin Province left 3.8 million people in Harbin, provincial capital of Heilongjiang, without drinking water for four days.

The 1,900-km long Songhua River is a major drinking water source for tens of millions of people in Jilin and Heilongjiang.

(Xinhua News Agency July 30, 2010)